This invention relates to a method and apparatus for removing an annular grooved tread from an initially closed segmented mold. One of the key problems in molding a large elastomeric tread is the removal of the finished product from the mold. Particular attention has to be focused on efficiently stripping the tread from the mold without damaging the finished tread. A second consideration in molding an annular tread is that the molded tread match new tire tread designs as closely as possible. A third design consideration is to minimize the amount of undertread thickness in order to make the tread more efficient in terms of rolling resistance.
In order to achieve the above-mentioned objectives, those skilled in the art have developed a variety of methods and apparatus for tread removal. Conventional strippers and knockoffs used in injection molding of rigid articles do not work with an elastomeric annular grooved tread.
Simply opening the mold in order to remove the tread cannot be accomplished without resulting in tread damage. The grooved tread tends to remain in the mold cavity, and by failing to release from the mold, the tread is stretched beyond its elastomeric limits. The excessive stretching results in the undertread deforming such that the annular shape is lost. This deformation makes the tread unsuitable for use.
Attempts to manually strip annular treads from a mold have been found to be too time consuming. This labor intensive method of tread removal is costly, and further, introduces human error that can result in tread damage.
In order to facilitate tread removal, artisans have attempted to minimize the number of grooves and have placed generous draft angles on those grooves used on a molded tread. In so doing, they have minimized their ability to match new tire tread designs. In the past, it was generally accepted practice to minimize tread removal problems by requiring a greater than 11/2.degree. draft angle. Grooves with draft angles less than 11/2.degree. greatly increased the force required to remove the tread from the mold. Additionally, a variety of mold release agents were used to assist in releasing treads from the mold. In the art of molding, the use of release agents introduces a foreign contaminant into the product and is generally avoided where possible.
Further design compromises were made by thickening the undertread of the molded article so that the product could take more abuse in order to release the tread from the mold. This resulted in an increase in material usage and a decrease in tread performance. The thicker undertread causes an increase in rolling resistance.
The method and apparatus of the invention allows an annular grooved tread to be removed from a mold in an efficient and reliable fashion and does not limit the design of the tread in terms of groove patterns or draft angles. Rather, it permits use of 0.degree. to 11/2.degree. draft angles on tread grooves and sipes.
Also, the invention enables the tread to be molded with a thin undertread, thus improving both the cost efficiency and the performance characteristics of the finished article.